The U.S. and other world powers have agreed on a plan with Iran to start rolling back parts of the Iranian nuclear program in exchange for some sanctions relief. Secretary of State John Kerry says the deal goes into effect later this month.

SECRETARY JOHN KERRY: As of that day, January 20th, for the first time in almost a decade, Iran's nuclear program will not be able to advance. In fact, parts of it will be rolled back while we start negotiating a comprehensive agreement to address the international community's concerns about Iran's nuclear program.

RATH: Kerry was speaking in Paris where he has a lot on his plate, including plans to hold a long-delayed peace conference on Syria next week. He's urging the Syrian opposition to attend the meeting and to negotiate with a regime they've been trying to topple. The war in Syria has raged for three years and has befuddled the international community. NPR's Michele Kelemen reports.

MICHELE KELEMEN, BYLINE: Jan Egeland describes Syria as the challenge for our generation, and that's something coming from a man who used to oversee U.N. aid operations around the globe.

JAN EGELAND: Since the turn of the millennium, we have not had such a massive displacement of people. Nine million and counting people have lost their homes. It is beyond anybody's imagination.

KELEMEN: Egeland, now secretary general of the Norwegian Refugee Council, says the international community is failing the people of Syria, but he's hopeful that in all the meetings this week, in Paris and at a donors conference Kuwait, countries will push Syria to give aid groups the access they need.

EGELAND: And there should be consequences for those who break these humanitarian principles, and we haven't seen that so far. So now, humanitarians are struggling alone, uphill. We are completely overwhelmed, especially inside of Syria where the aid operation is too weak, and the problems are too great.

KELEMEN: Secretary Kerry says 160,000 people are trapped, without aid, in a suburb of Damascus. He says there's an urgent need to focus on this and to get a long delayed peace conference off the ground.

KERRY: There is no other alternative to ending this violence and saving the state of Syria than to find a negotiated, peaceful outcome.

KELEMEN: Kerry is urging the fractious opposition to attend next week's conference in Switzerland, and he calls this a test of everyone's credibility. The president of the Syrian opposition council, Ahmad al-Jarba, isn't making any promises to attend just yet. But in Paris, standing alongside Kerry and other foreign ministers of countries that have been supporting him, the opposition leader said the goal must be a Syria without Bashar al-Assad. Jarba spoke through an interpreter.

AHMAD AL-JARBA: (Through translator) The most important aspect of today's meeting is that we all agree to say that al-Assad has no future in Syria. The al-Assad family has no future in Syria.

KELEMEN: And Jarba says there can be no ambiguity about that. Turkey's foreign minister Ahmed Davatoglu says he's hopeful Jarba will now decide to attend the conference, though he acknowledges it's a difficult decision as rebels fight extremists among their ranks and as Syria continues to bombard cities.

AHMED DAVATOGLU: The regime continued to attack in Aleppo by barrel bombs, continued to punish Syrian people by starvations. Therefore there is - they have some concerns about this. We will be doing everything possible to stop these massacres and crimes against humanity.

KELEMEN: It's not clear yet whether those assurances - heard many times before - will convince the Syrian opposition to attend peace talks or if negotiations can really succeed. Secretary Kerry is urging observers not to be too cynical and give these talks some time. Michele Kelemen, NPR News, Paris. Transcript provided by NPR, Copyright NPR.